As it is known by those skilled in the art, a lot of optical devices need one or more reflectors for providing their functionalities. This is notably the case of Fabry-Perot lasers, tunable lasers, RSOA (“Reflective Semiconductor Optical Amplifier”), reflective arrayed waveguide grating (or R-AWG), echelle gratings as well as more complex photonics integrated circuits (or PIC).
Actually, cleaved facets are the standard way to realize reflectors for integrated waveguides. Once these cleaved facets have received a coating they can provide a controlled level of reflection, because a fine tuning of the reflection coefficient can be set by the coating parameters (and notably the thickness and the type of material). However, this solution is not integrated and cannot be implemented within a PIC, for instance between a laser and a modulator inside an integrated laser modulator (or ILM). Furthermore, with the development of wide gain active components, broadband passive elements are now required in order to benefit from these active components. So, there is now a great demand for integrated broadband controlled reflectors. This is notably the case in widely tunable lasers where they may be used for the feedback control.
Two main solutions have been proposed for realizing integrated reflectors (or mirrors): bragg reflectors and reflective loops.
A bragg reflector consists in teeth etched in a chosen zone of a waveguide. When the index difference between the optical mode in the chosen etched zone and in the non-etched zone of the waveguide is large, a relatively broadband reflection can be achieved. However there are significant losses and a wavelength dependence that is an issue for obtaining a controlled reflection coefficient over a wide wavelength range.
A reflective loop can be realized with MMI (“MultiMode Interference”) coupler(s) or directional coupler(s). An example of reflective loop comprising a directional coupler is schematically illustrated in FIG. 1. This type of reflector allows the reflection/transmission ratio to be set by means of a judicious choice of the coupling coefficient, is easier to implement compared to a bragg grating because the critical dimension is relaxed, and induces very low loss and offers performances tolerant to fabrication variations. However, it does not show a broadband behavior because of the large variation of the coupling coefficient with the wavelength.